The present invention relates generally to plug valves and more particularly concerns a plug valve with a separate and discrete actuating stem which is useful in virtually eliminating any leakage of fluid from a process line. The invention will be disclosed, by way of example, in connection with an inverted plug valve having pressure chambers with disparate surface areas on opposite ends of the valve plug which serve to shift the valve in a direction traverse to the flow direction to reestablish a sealing relationship whenever leakage occurs. The invention includes a discrete and separate actuating stem for the plug to permit the plug to be axially shaftable in a piston like manner to sealingly interact with a packing disposed about the stem. In the preferred embodiment, this plug is pressure biased against the packing by a pressurized buffering media.
Considerable activity has been directed in recent years to the development of valves which use various types of non-metallic materials adapted to withstand the damaging effects of corrosive fluids, high pressures and various other elements encountered in the control of piped liquids and gases. The material employed is usually a gas and liquid impervious material such as TEFLON or an equivalent substance which provides an excellent seal under proper conditions of use, while at the same time offering minimal resistance to valve plug rotation, particularly in the case of larger valves.
Leakage may be characterized as either internal or external. Internal leakage occurs whenever fluid escapes past a closed valve member, and through the pipe or conduit which is being valved. External leakage is characterized by fluid which escapes to the atmosphere, usually through the valve member. It is obviously important to eliminate or reduce both types of leakage.
The need for leak-proof valves which eliminate atmospheric leakage has been highlighted recently due to sensitized concerns relating to ecology. There is thus a growing desire and need for valves which reduce atmospheric leakage to a nearly zero level and which also reduce internal leakage. In fact, in some instances and for some fluids, rigorous standards dictate the maximum permissable atmospheric leakage.
Leakage problems are frequently aggravated and compounded when the valve is subjected to temperature changes. The non-metallic sealing material generally has a much higher coefficient of expansion than the metals from which the main part of the valve is formed. As a result, the sealing material will expand far more and, under some circumstances, may even cold flow, that is, take a permanent set or change in shape or dimension. When subsequently subjected to cooler temperatures, these seals provide a relatively loose fit for the plug member.
One prior art solution to these problems in the past is illustrated in U.S. Pat. No. 3,472,485. This solution included the use of a valve plug with a separate actuating stem which permitted downstream shifting of the plug in response to line pressure if and when a sealing relationship was lost. U.S. Pat. No. 3,263,697 also permitted downstream shifting of the plug and, in addition, permitted transverse shifting of the valve actuating stem.
Another prior art solution has included the use of a high pressure inert buffering media at all potential leak paths of the valve. The buffering media is maintained at pressure which is in excess of the line pressure for the valved fluid to reduce leakage. One example of a method and apparatus for such pressure buffering of leak paths is shown in U.S. Pat. No. 4,116,208.
The present invention is in many regards similar to that disclosed in the aforementioned U.S. Pat. No. 3,263,697 but makes important changes thereover to further permit interaction of the actuating stem with a packing member to augment a seal to prevent external leakage.
Accordingly, it is an object of the present invention to provide a plug valve which effectively prevents both internal and external leakage.
It is a further object of the present invention to provide a plug valve which is pressure responsive and in which the valve stem moves relatively to the valve body to compensate for any loss in the sealing relationship between the valve stem and the valve housing.
It is still another object of the present invention to provide a plug valve with a discrete and separate activating stem which moves in a direction transverse to the fluid flow to sealingly engage an external sealing member.
It is still another object of the present invention to provide a plug valve which maintains an effective seal when subjected to temperature changes.
It is still another object of the present invention to provide a valve plug with an activating stem which moves axially with respect to the valve plug and the valve plug's rotational axis to compressingly engage an external sealing member in response to fluid leakage past an internal sealing member.
It is yet another object of the present invention to provide an actuating stem which is constantly urged to a sealing relationship against an external sealing member by a pressurized buffering media.